The low-density lipoprotein receptor mediates cholesterol homeostasis through endocytosis of lipoproteins. It discharges its ligand in the endosome at pH < 6. In the crystal structure at pH = 5.3, the ligand-binding domain (modules R2 to R7) folds back as an arc over the epidermal growth factor precursor homology domain (the modules A, B, β propeller, and C). The modules R4 and R5, which are critical for lipoprotein binding, associate with the β propeller via their calcium-binding loop. We propose a mechanism for lipoprotein release in the endosome whereby the β propeller functions as an alternate substrate for the ligand-binding domain, binding in a calcium-dependent way and promoting lipoprotein release.
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